A large fraction of the electricity use in the United States is generated by coal-fired power plants. Such combustion powered plants, and other emission sources, produce carbon dioxide in large amounts, and collectively in the billion ton scale. Effective materials and methods are required for the capture of carbon dioxide in order to reduce environmental pollution, and to comply with existing and proposed environmental laws and regulations. There exists a tremendous and unmet need for new technologies that effectively and efficiently separate carbon dioxide from other gases for environmental uses and reasons, as well as for any other application in which carbon dioxide is required to be separated and isolated from other gases.
For all those reasons, carbon dioxide capture from point sources like coal-fired power plants is considered desirable. However, separation of CO2 from flue gas to date has been difficult to accomplish in a cost-efficient, low-waste manner. Methods and sorbents with high gas selectivity, good chemical and thermal stability, low cost, and reversible adsorption are desired.
Flue gas emitted from coal-fired power plants constitutes 15-16% CO2, 6-7% H2O, 3-4% O2, and about 70% N21. Carbon dioxide capture from coal-fired power plant emitters is currently considered as a possible technology to stabilize the CO2 level in the atmosphere. A variety of sorbent materials are currently under investigation for carbon dioxide capture. Porous materials have been generally discussed as possible sorbents for carbon dioxide capture. Several classes of porous materials are currently under investigation, predominantly metal-organic frameworks (MOFs), activated carbons, molecular organic solids, and amine-functionalized silicas. More recently, research has begun on CO2 sorption involving covalent organic frameworks. Generally, sorbents which physisorb CO2 (e.g. MOFs and carbons) tend toward lower selectivity and sorption capacity at low CO2 pressure (≦1 atm) but exhibit facile reversibility. In comparison, strongly chemisorbing materials such as amine-functionalized silicas tend towards higher selectivities and capacities but typically show less facile reversibility. Recently, amine-functionalized MOFs have been reported that exhibit increased CO2 sorption at low pressure as well as significantly improved selectivity. Still, MOFs tend toward chemical instability (oxidation, hydrolysis) due to the dative nature of the metal-ligand bonds. It is apparent that inexpensive sorbents with enhanced chemical stabilities and heats of adsorption at the borderline between strong physisorption and weak chemisorption (ca. 25-50 kJ/mol) remain an attractive, yet unmet, target to provide adsorbents having the desirable properties identified herein.